1. FIELD OF THE INVENTION
The invention refers to a weft yarn detector in shuttleless weaving looms for detecting the presence or absence of the weft yarn in the shed after insertion of the weft yarn. It especially refers to weaving looms in which the weft yarn on the receiving side is clamped in a receiving gripper and moved together with this gripper towards the edge of the woven cloth in the beating up phase, wherein the correct position of the yarn in the receiving gripper is to be monitored.
2. DESCRIPTION OF THE PRIOR ART
An electromechanical sensor element is known from DE-Pat. No. 24 29 261, which for detecting the inserted weft yarn is integrated into a guide teeth mounted at the slay bar. This known sensor element is operated as follows: The inserted weft yarn is held and tensioned by said lateral receiving gripper while being beaten up. It is moved together with the receiving gripper towards the edge of the woven cloth. Simultaneously, the slay bar and the reed also are moved in the same direction. Thereby, the known sensor element is passed under the inserted weft yarn thereby contacting the same. However, since the sensor element and the weft yarn are moved substantially in the same direction there is an only small relative movement leading to small sensor signals, which are difficult to be discriminated from noise signals. Moreover, it can not be excluded that the sensor element also is contacted by warp threads. Unjustified machine stops or non detected faults may result therefrom. A further severe disadvantage of the known sensor is its location at the receiving side end of the slay bar. Since it replaces guide teeth, the guide characteristics for the projectile may be influenced near the receiving side of the slay bar. Moreover, this side of the slay bar is especially exposed to damage due to clamped projectiles and there is a danger of destruction of the expensive sensor. The electrical connection of the known sensor is complicated and expensive because it is arranged on a moving part of the weaving loom. Finally, one of the most important deficiencies of this sensor is its limited measuring range, since it can detect the presence or absence of the inserted weft yarn only in a very early phase of the beat-up motion. All faults happening in a later phase of the beat-up procedure are not detected.